1. Field of the Invention
The present invention relates to a non-insulated step-down switching regulator (DC/DC converter).
2. Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators, etc., or otherwise electronic devices such as laptop computers, cellular phone terminals, and PDAs (Personal Digital Assistants), each mount a switching regulator which steps down a DC voltage to a voltage level that corresponds to a load.
FIG. 1 is a circuit diagram showing a configuration of a non-insulated step-down switching regulator investigated by the present inventors. A switching regulator 2r steps down a DC input voltage VIN input to its input terminal P1, and outputs the voltage thus stepped down to its output terminal P2.
The switching regulator 2r includes a switching transistor M1 configured as an N-channel MOSFET, a rectifier diode D1, an inductor L1, an output capacitor C1, a current detection resistor Rs, a control circuit 100r, and a feedback circuit 102.
One end of the current detection resistor Rs is connected to the switching transistor M1. The other end of the current detection resistor Rs is connected to the cathode of the rectifier diode D1. One end of the inductor L1 is connected to the cathode of the rectifier diode D1. The other end of the inductor L1 is connected to the output capacitor C1.
The control circuit 100r is configured as a control circuit employing a current mode pulse width modulation method or otherwise a voltage mode pulse width modulation method. The control circuit 100r includes a current detection terminal CS, a switching terminal OUT, a feedback terminal FB, and a ground terminal GND. The control circuit 100r is arranged such that its switching terminal OUT is connected to the gate of the switching transistor M1, its ground terminal GND is connected to the cathode of the rectifier diode D1, and its current detection terminal CS is connected to one end of the current detection resistor Rs. During the on period of the switching transistor M1, a coil current IL flows between the current detection resistor Rs and the inductor L1. In this state, a voltage drop (detection voltage Vs) occurs across the current detection resistor Rs in proportion to the coil current IL. The detection voltage Vs is fed back to the current detection terminal CS.
The feedback circuit 102 generates a feedback voltage VFB according to the output voltage VOUT of the switching regulator 2r, and inputs the feedback voltage VFB thus generated to the feedback terminal FB of the control circuit 100r. The feedback circuit 102 includes a photocoupler, for example. The feedback circuit 102 functions as an error amplifier which generates a feedback voltage VFB according to a difference between the output voltage VOUT and a target value.
The control circuit 100r generates a pulse signal SPWM having a duty ratio which is adjusted such that the output voltage VOUT matches the target value according to the feedback voltage VFB while maintaining the coil current IL at a constant level according to the detection voltage Vs.
Specifically, the control circuit 100r turns on the switching transistor M1 for every predetermined period. When the switching transistor M1 is turned on, the coil current IL of the inductor L1 increases with time. The detection voltage Vs increases according to an increase in the coil current IL.
When the detection voltage Vs reaches the feedback voltage VFB, i.e., when the coil current IL reaches a current value which is adjusted according to the output voltage VOUT, the control circuit 100r turns off the switching transistor M1. The control circuit 100r repeatedly performs the aforementioned operation, thereby switching on and off the switching transistor M1.
With the switching regulator 2r shown in FIG. 1, during the off period of the switching transistor M1, charge is stored in the drain-source capacitance of the switching transistor M1. When the switching transistor M1 is turned on, the charge stored in the drain-source capacitance is instantly discharged. In this case, spike current flows through the current detection resistor Rs. This leads to overestimation of the coil current IL that flows through the inductor L1. As a result, immediately after the switching transistor M1 is turned on, and before the coil current reaches a sufficiently increased value, the switching transistor M1 is turned off, which is a problem.
In order to prevent misdetection of the coil current IL due to such a spike current, a method is conceivable in which, during a predetermined mask time TMSK immediately after the switching transistor M1 is turned on, the detection voltage Vs is masked. With such an arrangement employing such a mask time TMSK, the switching transistor M1 is not switched to the off state, i.e., the switching transistor M1 is maintained at the on level, during at least the mask time TMSK for each cycle. That is to say, the mask time TMSK functions as the minimum on time of the switching transistor M1.